CDMA is a kind of modulation and multiple-access connection technology based on spread spectrum communication. In the spread spectrum communication technology, a digital signal is multiplied by a pseudo random code with high bit rate at the signal transmitting end. Since bit rate of the pseudo random code is much higher than that of the digital signal, the signal transmission bandwidth is spread. At the receiving end, the received signal is multiplied by the same pseudo random sequence to make the correlation operation and the spread signal is de-spread.
Nevertheless, in CDMA system, the pseudo random codes are not orthogonal perfectly, and this causes mutual interference among different channels and among different subscribers. Since CDMA system is a self-interference system, in order to prevent near-far effect from appearing and signal-to-interference ratio (SIR) from being changed, an inner loop power control method and an outer loop power control method are simultaneously employed for uplink and downlink. The result of power control is that the amount of transmission power reflects the condition of connection between system and service, such as a cell load, a channel condition and path loss of the propagation environment etc.
When the propagation condition is changed, for example air interface interference is increased, or a subscriber is at the cell boundary that is farther apart from the base station and the path loss is increased, in order to make the signal at receiving end have a certain SIR, the transmission power allocated to each subscriber needs to be increased. This causes the transmission power of mobile station and base station to be increased too. Increase of transmission power will increase air interface interference, and more interference on air interface will need more transmission power. This is a positive feedback procedure, and the transmission power is getting higher and higher. When the system load has reached a certain degree, this positive feedback will make the transmission power increase rapidly at the mobile station and the base station, and finally the system load will reach near the saturation state.
In general, during the procedure of establishing or re-configuring channel, the system allocates a transmission power range, which corresponds to the resource allocated to the link, to the mobile station. This is an admission control for a service request before a channel has been established. Therefore, it is considered that the system does not crush when transmission power is within the configured range. In contrast, when the system load is near the saturation state, it is possible that the transmission power of a channel is out of the configured range; in this case, it is necessary to have a negative feedback control mechanism to reduce the system transmission power to compensate the positive feedback of increasing power. In this invention, provides a method that decreases a channel transmission power to reduce the system load, and reducing the channel rate within the QoS allowable range.
When cell load increases or channel condition becomes worse, usually QoS for a subscriber with high service rate is decreased more rapidly than that for a subscriber with low service rate; this is another reason to decrease the service rate in this situation. This situation may be explained with an example of an Adaptive Multi-Rate (AMR) voice service. FIG. 5 shows the Mean Opinion Score (MOS) to Carrier to Interference (C/I) curves for different AMR modes. It can be seen from FIG. 5 that when the C/I becomes smaller, i.e. the cell load becomes heavier, the channel condition becomes worse and the mobile station goes farther from the base station, the MOS of a higher rate AMR mode decreases more rapidly than that of a lower rate AMR mode. This means that a lower rate service will have better QoS than a higher rate service. If the rate remains unchanged, QoS for subscribers will decrease rapidly. Therefore, in this case, decreasing the rate will improve the QoS relatively.
In contrast, when cell load becomes lighter, QoS for a subscriber with a higher service rate is increased more rapidly than that for a subscriber with a lower service rate. In a condition that cell load becomes heavier, decreasing service rate in a certain range can slow the decreasing rate of QoS, but the QoS has been decreased already. When resource is available, increasing service rate can increase QoS; so we should transmit the voice and data with the as rapid rate as possible. When system load becomes lighter, it is necessary to increase rate appropriately so as to provide better QoS.
The U.S. patent named “Code division multiple access system providing load and interference based demand assignment service to users” (U.S. Pat. No. 6,088,335) discloses a method for allocating rate. The invention has the following features. At first, the rate to be increased is decided according to pilot signals. Secondly, the rate is decided according to the difference between the maximum pilot signal in active set and that in non-active set. Thirdly, a series of threshold levels are set and each threshold level corresponds to a rate. At last, another method is provided, in this method, the rate is decided according to the average capacity load of adjacent cells and the signal strength of received pilot signals. There are several problems in the technical scheme disclosed in the U.S. patent. First, the pilot data do not directly reflect change of service rate; it is difficult to response the effect of rate adjustment rapidly. Secondly, with this method the uplink rate and downlink rate cannot be controlled separately because the pilot data only reflect the downlink load condition. In CDMA system, uplink service and downlink service is unsymmetrical, and it cannot satisfy service requirement if uplink rate and downlink rate cannot be controlled separately. Furthermore, it is impossible to deal with the situation that uplink rate and downlink rate need to be adjusted in different direction, such as one should be increased and another should be decreased.